This invention relates generally to noise reduction panels and, more particularly, to a method and system for improving a vibratory response of noise reduction panels.
At least some known acoustic panels used to line the fan flowpath of a turbine engine for noise reduction may be exposed to a high vibratory forcing function, much of which can be due to the aero shock waves of fan blade passing. Initial design intent is to make the panels and their supporting structure stiff enough so that they do not respond to this stimulus. Weight and/or maintainability design constraints sometimes undermine this design intent. For example, a bolted-on panel is preferable to a panel that is bonded to the fan case for maintainability, allowing easy replacement of damaged panels in service. Also, to reduce weight, panel section properties may be minimized. These added design constraints may reduce the installed panel stiffness, causing it to have a small frequency margin from the driving excitation. This may result in a forced vibratory response that may cause excessive alternating stress in the panels and/or its supporting fasteners.
For example, a forward acoustic panel of some known engines is a composite laminate structure that is bolted to a radially inner surface of the engine fan containment case, just forward of the fan blades. The forward end of each panel is supported by bolts that span the arc covered by the panel. The aft end is supported by insertion of a lip formed in the panel into a mating groove of the fan case. Between these supports the panels are free to vibrate, restricted only by elastomeric spacers bonded to the outer surface of the panel and residing in the small radial gap between the panel and the inner surface of the fan case. When the vibration amplitude of the unsupported portion of the panels exceeds the gap between the spacers and the fan case, the spacers act as springs in compression and add stiffness to the overall panel. However, since the spacers have very little damping, they act as almost purely elastic springs, dissipating very little vibrational energy. The overall affect of the spacers is not enough to make the panel unresponsive to blade passing stimulus.